This invention relates to inorganic phosphors. In particular, this invention relates to inorganic phosphors having a diamond-like carbon coating and to the method of making these diamond-like carbon-coated inorganic phosphors.
Phosphor particles are used in a variety of applications such as flat panel displays and decorations, cathode ray tubes, and fluorescent lighting fixtures. Luminescence or light emission by phosphor particles may be stimulated by the application of heat (thermoluminescence), light (photoluminescence), high energy radiation (e.g., x-rays or e-beams), or electric fields (electroluminescence).
Electroluminescent inorganic phosphors are of particular commercial importance. They are used in electroluminescent lamps, which in turn are used in, e.g., watches, clocks, and communication devices. The luminescent brightness and maintenance of the brightness of such inorganic phosphors are two important criteria for characterizing phosphor particles. Inorganic phosphors are subject to degradation which causes them to lose their brightness, and thus shortens their lifetime. Luminescent brightness is typically reported as a quantity of light emitted by the subject phosphor when excited. When reporting brightness (also referred to as luminosity), the conditions under which the phosphor is excited should also be reported. This is because the value depends on several factors including the voltage and frequency of the applied electric field and the temperature which the phosphor experiences. Maintenance refers to the rate at which inorganic phosphors lose brightness during operation. Water vapor is one of the most important adverse influences on maintenance. The effect of moisture or high humidity is referred to herein as humidity accelerated decay.
One way to protect inorganic phosphors and slow the rate of humidity accelerated decay is to encapsulate them with inorganic coatings, e.g., oxide coatings. Such coatings are generally transparent in order to prevent the loss of light emission by the inorganic phosphors and comprise oxides such as silica, titania, alumina, and mixtures of these. These coatings have been deposited by chemical vapor deposition onto phosphor particles in a fluidized bed. U.S. Pat. Nos. 5,156,885, 5,418,062 and 5,439,705 (Budd) describe encapsulated electroluminescent inorganic phosphors which exhibit high initial luminescent brightness coupled with resistance to humidity-accelerated decay.
Although there have been advances in the art of coated phosphor particles, there is still room to improve the lifetime and luminescent brightness of inorganic phosphors, particularly in humid environments. The deposition of diamond-like carbon coatings onto phosphor particles to achieve this objective has not previously been demonstrated.
In one aspect, this invention is an inorganic phosphor particle, wherein a diamond-like carbon coating is on at least a portion of the surface.
In a preferred embodiment, the inorganic phosphor particle further comprises a transparent layer of one or more organic or inorganic materials between the particle surface and the diamond-like carbon coating.
In another aspect, this invention is a method of coating diamond-like carbon onto inorganic phosphor particles comprising:
providing a multiplicity of inorganic phosphor particles;
forming a plasma from a carbon-containing source comprising reactive species in proximity to the multiplicity of inorganic phosphor particles;
exposing the multiplicity of particles to the reactive species in the plasma;
whereby deposition of diamond-like carbon onto at least a portion of the surface of the inorganic phosphor particles occurs.
In a preferred method, energy is capacitively coupled into the plasma.
In a most preferred method, the conditions present in the capacitively coupled system further comprise an ion sheath.
As used in this application, xe2x80x9cdiamond-like carbonxe2x80x9d refers to an amorphous film or coating comprising approximately 50 to 90 atomic percent carbon and approximately 10 to 50 atomic percent hydrogen, with a gram atom density of between approximately 0.20 and approximately 0.28 gram atoms per cubic centimeter, and composed of approximately 50 to approximately 90% tetrahedral bonds.
As used in this application, xe2x80x9camorphousxe2x80x9d means a randomly-ordered non-crystalline material having no x-ray diffraction peaks.
As described herein, the present invention has several advantages. The present inventors have found that diamond-like carbon coated onto inorganic phosphors, and particularly onto oxide-coated inorganic phosphors, imparts a high initial luminescent brightness and surprisingly high resistance to humidity accelerated decay. Diamond-like carbon coatings are desirable because they provide both chemical and mechanical protection to a substrate. DLC-coated inorganic phosphor particles are useful because they resist degradation at elevated temperatures and humidity. Electroluminescent lamps made with DLC-coated inorganic phosphor particles maintain a higher level of brightness, or luminosity, over longer periods of time than similar lamps made with oxide(only)-coated phosphor particles. Comparisons have shown a six-fold improvement in Time to Half-Life of the DLC-coated inorganic phosphors over oxide(only)-coated inorganic phosphor particles.
The methods disclosed, in particular the preferred methods, provide a fast and efficient process for depositing densely-packed diamond like carbon coatings onto inorganic phosphor particles.
Other advantages of the invention will be apparent from the following description, figures, examples, and appended claims.